Micrologix 1400 Modbus Issues

julianf400

Member
J
Join Date
Jun 2008
Location
Melbourne
Posts
10
Hi All,

I am currently working on a project where i need to send read messages via RS485 modbus, the micrologix is the master and there are 2 Munters dehumidifier units as slaves and have come pre-addressed with 4800/001 in the settings.

Now i have managed to get communications working by changing the baud to 19200 and addressing them as 002 & 003. will this affect my comms if there is slave address 001 missing in the network?

i am using approx 4-5 msg instructions per unit and only the 1st message in the staggered setup is not working or works intermittantly...any suggestions?
 
how fast are you polling the units? how are they wired? daisy chained or star?

do you have a 120 ohm terminating resistor?
 
There's no requirement for Modbus slave node numbers to be sequential or to start at 1, and there is no "polling list".

Are you getting actual error messages, or just Timeouts ?
 
yes i know the typical network is of daisy chain configuration but in this case the PLC is in the middle of the installation and 2 cables are parralelled up at the PLC adaptor and both are fitted with resistors, the messages are only timing out.
 
There's nothing wrong with the PLC being in the middle of a "star".

RS-485 systems can be tricky to wire up, depending on how the common reference is connected and how the terminations are done.

I presume you are using the built-in RS485 on the MicroLogix 1400's Channel 0 connector.

How long is the wire to each device ? What type of wire are you using ?

Where, exactly, are the termination resistors ? What value are they ?

Do these Modbus slave devices have a dedicated Common terminal, or do they connect to Earth Ground and just have Data A and Data B wires ?
 
Ken,

i am using the built in RS485 on the micro via channel 0 using the 1763-nc01 adaptor. the resistors are both 120 ohm and the cable being used is a 2 pair Belden cable with only 2 pair being used, the distance would be approx 50-60 yards. i believe the shield at the slave end to be referenced to ground but i will check today.

as for there being no slave address of 001 will this cause problems?
 
I bet...because it happenned to me

Try a delay between drives. If you follow the AB example right from the AB manula for ML1400, it won't work. The second drive will always fault an comm errors. Add a delay of 0.10sec between read / write sessions with each drive....something like that... Try with just one drive connected...I think it is a comm error but not an RS-485 electrical defficiency.
 
Last edited:
Sixty yards between devices with a ground return path, plus an unknown shield configuration, says "signal degradation" to me.

It can't hurt to put in delays between MSG instructions, especially if that also leads the programmer to trap the Error codes on the MSG instructions.
 

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